Our Milky Way's halo of hot gas is warmer to the 'south' than the 'north' because of an internal combustion engine-like effect that is compressing the gas like a piston, a new study has found.
Computer simulations reveal that the Large Magellanic Cloud – a satellite galaxy below, or on the south side, of our own – attracts the Milky Way, causing gas in the southern half of the halo to compress and heat up.
This, a team of scientists led by the University of Groningen say, explains why the southern half of the halo is up to 12 per cent warmer than the northern part above the Milky Way's disc, a discrepancy which was measured in 2024 by the X-ray observatory eROSITA mounted on a German-Russian space telescope.
Their findings are published today in Monthly Notices of the Royal Astronomical Society.
Many galaxies, including our own, are surrounded by a vast sphere of thin and warm matter, also known as a halo of hot gas.
Scientists estimate that our Milky Way's gaseous halo has a mass of 100 billion solar masses, meaning there is more matter in the halo than in the galactic disc. The halo, which has a temperature of about 2 million degrees kelvin (a few hundred times hotter than the surface of the Sun), is the 'building material' of the much more compact and cooler disc of gas and stars – including the Sun – at the centre of it.
The Milky Way in the computer simulations is made of three 'components': the rotating disc with relatively cold gas, the much warmer gas around it and a large halo consisting of dark matter. The so-called hydrodynamic simulation calculates movements of these three components caused by the gravitational attraction of the Magellanic Clouds, which are passing close by the Milky Way, over the course of about one billion years.
The results show that the Milky Way's cold disc is currently moving towards the satellite galaxies at about 40 kilometres per second because of the gravity of the Large Magellanic Cloud. In this process, the Milky Way compresses the gas at the bottom and the material heats up 13 to 20 per cent, according to the calculations.
The simulation also shows that the temperature difference between the northern and southern parts of the halo has arisen in the last 100 million years.
"We saw fairly quickly in the simulations that there was a warming effect," said Filippo Fraternali, professor of gas dynamics and the evolution of galaxies at the University of Groningen.
"It took a little longer before we realised what is going on here – namely the compression of gas like in the piston of an internal combustion engine, which then heats up to make the southern side of our Milky Way's halo warmer."
The simulations may also explain more asymmetries around the Milky Way, according to the researchers. For example, many more so-called high-velocity clouds are seen on the north side of the Milky Way than on the south side. These regions of gas – usually about 100 times cooler than the surrounding material – move around the galaxy at highly anomalous speeds.
"The lower pressure of the surrounding gas may make it easier for these clouds to form and survive there," Fraternali added.
Initially, the researchers were not looking for what they discovered. The simulations had already been published in 2019 as part of an attempt to find an explanation for gas moving around the Magellanic Clouds, among other things. At that time, the temperature difference had not yet been found.
"Typically, computer models are designed to explain certain observations. It is remarkable these simulations already contained the temperature asymmetry before it was found. It makes this result extra robust," Fraternali said.
Co-author Else Starkenburg, associate professor at the University of Groningen, added: "Our explanation for the temperature asymmetry measured by eROSITA is based on simple and well-understood physical processes as we also find them in, for example, combustion engines.
"That gives the result extra elegance."
ENDS